1. Field of the Invention
The present invention relates to a radio terminal and a transmission power control method employed in a radio communications system.
2. Description of the Related Art
Lately, radio communications systems that can achieve high communication speeds between a base station (BS) and a mobile station (MS) (radio terminal) have been proposed or standardized because of technological progress in radio communications.
Examples of these radio communications systems include Mobile WiMAX (Worldwide Interoperability for Microwave Access) (hereinafter referred to as WiMAX), which has been standardized as IEEE 802.16e in recent years. In WiMAX, Orthogonal Frequency Division Multiple Access (OFDMA) is adopted, and it is possible to increase communication speed by using a large number of subcarriers modulated at a high coding rate. Examples of modulation systems include 64QAM (Quadrature Amplitude Modulation). Further, WiMAX achieves sharing the same frequency band between multiple radio terminals (MSs). For example, according to WiMAX, a frame is divided into regions called “bursts” determined by both Frequency Division Duplex (FDD) and Time Division Duplex (TDD).
WiMAX adopts such a multiplexing system. A BS simultaneously receives electric waves from multiple MSs, and demodulates them at the same time. Therefore, the reception levels of the electric waves from the MSs are desired to be uniform at a certain level in the BS. Transmission power control (TPC) is commonly applied in order to make the MS reception levels uniform at a certain level in the BS. TPC is a control protocol that controls transmission power between the BS and MS. The MS controls power suitably based on TPC. Here, when TPC is applied in mobile communications, the MS may be positioned near or far from the BS. In order for the MS to have appropriate power for reaching the BS in such an environment, it is desirable that the MS can control transmission power level continuously over a wide range.
For example, the MS includes multiple variable gain amplifiers, and has these variable gain amplifiers arranged at cascade connection. Thereby, the MS can widen a power control range and therefore can have a wide output range.
Reference may be made to Japanese Laid-Open Patent Application No. 2001-358601 for related art.
However, it is costly in terms of components to provide an amplifier or attenuator having a mechanism for continuously varying output gain. Accordingly, in the case of providing the MS with two amplifiers, one of the two amplifiers may be a dual gain amplifier which has a varying mechanism simple enough to be able to switch gain between two levels. Thereby, it is possible to reduce component cost.
For example, in the case of using a transmission amplifier that includes a continuously variable gain amplifier whose range of gain control is 40 dB and a dual gain amplifier which is capable of switching attenuation between 0 dB and −20 dB, a maximum gain control width (range) of 60 dB can be obtained.
The above-described background art, however, has problems such as the following.
A description is given of the case where the MS has two variable gain amplifiers and there is a large difference in levels between which the gain is switched in one of the two variable gain amplifiers.
In this case, a discontinuity in transmission power is prone to occur at a gain switch point in the variable gain amplifier having a large difference in levels between which the gain is switched.
By way of example, a description is given of the case of obtaining a transmission power level lower by 30 dB than a maximum gain. In this case, the following two setups may be chosen: one is to set the gain of the dual gain amplifier at 0 dB and reduce the gain by 30 dB in the amplifier capable of continuously varying the gain, and the other is set the gain of the dual gain amplifier at −20 dB and reduce the gain by 10 dB in the amplifier capable of continuously varying the gain. However, there may not be matching in the difference in attenuation of 20 dB at the continuously variable gain side and the difference of the gain between the low gain and the high gain at the dual gain amplifier because of the individual differences of components and environmental factors such as a difference in temperature. In this case, a step occurs in transmission power at the point of switching gain distribution.
In order to control this step in transmission power with such a technique as adjustment, a circuit for calibration may be added. This calibration circuit makes adjustments so as to make power uniform at two operating points. However, addition of such a calibration circuit, which requires implementation of an additional circuit and control process, is not desirable.
If the gain were switched in a continuously variable manner in each of the two variable gain amplifiers, such a point of discontinuity could be eliminated by performing such control as to control the gain of one of the two variable gain amplifiers while fixing the gain of the other one of the two variable gain amplifiers.
However, as previously mentioned, it is costly in terms of components to provide an amplifier or attenuator having a mechanism for continuously varying output gain.